Why do pump motors fail? In 1987, the publishers of the Electric Motor Manual studied 9,000 motor failures and categorized the failures into seven types – overload, single-phasing, old age, bearing failures, rotor failures, contaminants and miscellaneous. The interesting thing about this study is that all of these failed motors were protected by fuses or circuit breakers for shorts, and by overload heaters for overload, unbalance and single-phase faults. So, why didn’t the fuses and overloads in the three-phase pump control panel protect the motors?
The primary answer is response time. For instance, a typical overload relay (see Figure 1) can take up to 20 seconds to respond to a single-phase condition. Imagine the heat build-up in the one remaining winding in a single-phased motor having to carry the entire load by itself. If it doesn’t fail immediately, its life will have been substantially shortened by the damage to the winding insulation, caused by overheating. A conventional overload relay responds slowly to single-phase conditions because it is reacting to the effect of the single-phase condition, which is heat, not to the actual occurrence of the single-phase condition.
Operating a three-phase motor on unbalanced power also can cause serious overheating that will dramatically shorten its life. Figure 2 shows the effect of voltage unbalance on motor life.
Figure 3 is the horsepower de-rating curve related to voltage unbalance. For instance, if you had a 71⁄2-HP load and 5-percent voltage unbalance, it would be necessary to use a 10-HP motor to prevent premature motor failure.
Solid-state phase monitors, as opposed to overload relays, detect faults like single-phasing, voltage unbalance, and high and low voltage electronically. Instead of taking up to 20 seconds to respond to a single-phase condition, as with a mechanical overload relay, they can respond immediately to these conditions. A single-phase condition occurs, and click, in less than a second, the motor is turned off. Or, if the single-phase condition occurs while the motor is off, the phase monitor will prevent the motor from coming on until the condition is corrected. The same holds true for reverse-phase conditions and severe voltage unbalance as well.
Three-phase protection devices can be divided into three categories – phase monitors (also called voltage monitors), current monitors and multi-function devices.
Voltage MonitorsVoltage monitors not only turn off pumps when the voltage is outside of pre-set, safe parameters, they also prevent them from starting if voltage conditions are not right. In this regard, they offer better protection than current monitors, which require that the pump run for a few seconds before the current responds enough to the voltage problem to trip out the motor. In a single-phase or severe unbalance condition, this might damage the motor. In a reverse-phase condition, the pump could be damaged by running backward for a few seconds.
Voltage monitors are relatively inexpensive, and provide protection for single-phase, reverse-phase and phase unbalance faults, plus over and under voltage. Some also include a power-up delay timer, which is necessary in a line shaft turbine application, to prevent the pump from restarting for a minute or so after a power interruption while the turbine is backspinning.
Their drawback is that they ignore well-related problems because they don’t monitor current. Their best use in a pump application is to supplement a current monitor that does not have built-in voltage protection. I always recommend both voltage and current monitors, or a combination multi-function unit, for any motor larger than 15 HP.
Current MonitorsCurrent monitors compare the current going to the pump with pre-set, safe levels, and turn the pump on or off accordingly. They read the current indirectly, using a device called a current transformer (CT). A CT is a coil about the size of a donut, through which a conductor runs, producing a secondary current proportional to the current flowing through the conductor. This secondary current then is read by the current monitor. The CTs can be built into the current monitor, or can be mounted externally in the pump control panel.
Most current monitors use three CTs – one for each phase – and look for over-current, under-current, current unbalance, single-phase and reverse-phase faults. Most are field-calibratable. If you are not relying on the current monitor to detect unbalance, single- and reverse-phase faults because they are unlikely to occur in your local area, or because you already have a voltage monitor to perform these functions, a single-coil current monitor can be used to monitor over-current and under-current, saving some money.
Multi-function DevicesMulti-function devices combine several functions in one package. For instance, Coyote’s three-phase units are available with a built-in contactor that will handle up to 40 HP at 460 volts. Symcom’s Model 777 combines a voltage monitor, current monitor, power monitor and ground fault monitor into one unit. The 777 is available in several configurations, including one that monitors the motor winding temperature for applications with motors equipped with a temperature sensor. Franklin’s SubMonitor also has this feature.
High Voltage Surge ProtectionFinally, a word about voltage surges and lightening strikes: Solid-state motor protection devices are like any other piece of electronic equipment in that they are vulnerable to high voltage surges. Most have built-in surge protection to handle small, short duration surges and voltage spikes, but if you are in an area where large voltage spikes or lightening strikes are possible, the addition of lightening arrestors to protect your protector is recommended. If your protector does get taken out by a voltage surge, chances are, it will have saved your motor by interrupting the power on its way to the grave.
In SummaryTo summarize, the motor protection provided as standard in a pump control panel will not always protect the motor from being damaged by electrical and load-related anomalies. The use of additional protection, such as electronic voltage monitors, current monitors and multi-function devices, is cheap insurance to protect your pump motor and help it give you years of trouble-free service.
The following companies advertise three-phase pump protection devices in this and other ground water publications.
7120 W. 117th Ave. Unit B4
Broomfield, Colo. 80020
400 E. Spring Street
Bluffton, Ind. 46714
Load Controls Inc.
53 Technology Park Road
Sturbridge, Mass. 01566
2880 N. Plaza Drive
Rapid City, S.D. 57702